1. Field of the Invention
The present invention relates to an image sensing apparatus, and more particularly to an image sensing apparatus having an image sensing device including a photoelectric conversion device section, vertical shift registers, and a horizontal shift register.
2. Related Art Statement
Various types of image sensing apparatuses converting subject images optically formed into electric signals have been proposed. These types of image sensing apparatuses each have, for example, an image sensing device including a photoelectric conversion device section, vertical shift registers, and a horizontal shift register. Then, using this image sensing device, the image sensing apparatus converts the above-mentioned optical subject image into an electric signal.
Although many types of these image sensing devices are known, for example, the construction of a vertical overflow drain type CCD will be described with reference to a block diagram of an embodiment according to the present invention that is shown in FIG. 1. In addition, here, an interline transfer type CCD will be described.
This CCD comprises: photodiodes 1 that are arranged two-dimensionally in a horizontal direction and a vertical direction and store electric charges by receiving light; vertical shift registers 2 that receive the electric charges stored in these diodes 1 through transfer gates not shown and thereafter transfer them in a vertical direction in order; a horizontal shift register 3 that sequentially transfers in a horizontal direction the electric charges transferred from these vertical shift registers 2; and a signal detector 4 that amplifies and outputs an output signal from the horizontal shift register 3.
FIG. 2 also relates to an embodiment of the present invention, and a block diagram showing the construction of an image sensing apparatus having a CCD like the CCD shown in FIG. 1. The construction of the image sensing apparatus will be described with reference to FIG. 2.
This image sensing apparatus comprises:
a lens 11 forming a subject light beam on a light receiving plane of a CCD 13 described later; a light shading element 12 composed of, for example, a mechanical shutter controlling whether the element permits the subject light beam from this lens 11 to pass or not; a CCD 13 converting into an electric signal the subject light beam passed through the light shading element 12; a signal processing circuit 14 outputting as a video signal 15 the electric signal from this CCD 13 after performing various processing on the signal; a driver 16 controlling the light shading element 12; a signal generator 17 supplying pulses for controlling a signal charge storage period in the photodiodes 1 of the CCD 13, pulses for driving the vertical shift registers 2, pulses for the horizontal shift register 3, and pulses for driving the signal processing circuit 14 so as to synchronize it with the CCD 13; and a CPU 18 totally controlling each of the circuits including the driver 16 and the signal generator 17. In addition, the signal processing circuit 14 and the signal generator 17 are built in a digital signal processor (called a DSP in figures) 19.
Next, each signal at the time of driving an image sensing apparatus, having the construction as described above, with related art is expressed in a timing chart shown in FIG. 10. FIG. 10 shows a vertical sync signal VD, transfer gate pulses TG, sub-pulses SUB, vertical shift register transfer pulses VT1, VT2, VT3, and VT4, clamp pulses CLP, a light shading element, and a CCD signal.
The vertical sync signal VD is composed of a pulse train (here, defined as a frame) defining predetermined unit periods for each obtaining a signal expressing an image, and respective periods defined by respective pulses are called V1, V2, V3, V4, V5, V6, V7, V8, and so on.
The transfer gate pulses TG are the pulses determining the timing when electric charges stored in the photodiodes 1 are transferred to the vertical shift registers 2. Further, respective pulses referred to as TG0, TG1, TG2, TG3, TG4, TG5, TG6, and TG7 are outputted synchronously with respective pulses defining the periods V1, V2, V3, V4, V5, V6, V7, and V8.
The sub-pulses SUB, in this vertical overflow drain type CCD, are the pulses for vertically discharging the electric charges generated in the photodiodes 1. The electric charges are discharged while the sub-pulses are outputted, and the electric charges are stored in the photodiodes 1 when the pulses are stopped during the time intervals tb1, tb2, tb3, tb4, tb5, tb6, and tb7. Furthermore, by controlling this charge storage time, a so-called device shutter controlling effective exposure time is achieved. Still further, the charge storage time in the photodiodes 1 is determined on the basis of the result obtained by performing photometry of a subject image with photometry means not shown. This charge storage time is controlled by these sub-pulses which further fragment the one frame period.
The vertical shift register transfer pulses VT1, VT2, VT3, and VT4 are the pulses for driving the vertical shift registers 2 and thereby sequentially transferring the electric charges to the horizontal shift register 3.
The clamp pulses CLP are the pulses for clamping an optical black portion of an output signal from the CCD 13, which stabilize the potential level of a video signal and keep a black level stable.
The light shading element 12 normally open and, after electric charges are stored in the photodiodes 1 by a record trigger, the element 12 shades light when the electric charges are transferred.
The output signal from the CCD 13 is composed of optical black portions in the vertical direction as shown in time intervals to1 and to2, and effective periods each of which is the period of the subject image sandwiched by the optical black portions. Further, usually, the signal level in the effective period is higher than the signal levels in the optical black portions.
Conventionally, when image-sensing is actually performed, vertical shift register transfer pulses VT1, VT2, VT3, and VT4 are outputted in succession if a record trigger is issued, for example, by pressing an image-sensing button and the like in the period V3. Owing to the transfer pulses, unnecessary electric charges in the vertical shift registers 2 are discharged at high speed as shown in a time interval ta.
After that, in a period V4, electric charges are stored in the photodiodes 1 on the basis of a predetermined exposure period included in a time interval tb4 of the sub-pulses SUB, that is, this time interval tb4 becomes an exposure period of one frame of image.
The image exposed in this manner in the time interval tb4 within the period V4 is outputted in a period V5 as a signal CS4. This signal CS4 is a CCD signal that is outputted from the signal detector 4 as the result of exposure caused by the record trigger.
In the period V5 when this signal of the CCD 13 is read, the light shading element 12 is closed after the time interval of closing operation, tm so that light may not reach the CCD 13. Then, this light shading element 12 is opened by the driver 16 driving the light shading element again in and after the start of subsequent period V6.
The image received by the CCD 13 during the period V5 when this light shading element 12 is closed is outputted in the period V6 as a signal CS5. Since this signal CS5 is a CCD signal in the period when the light is shaded by the light shading element 12, the signal levels in the optical black portions are approximately equal to the level in the effective period.
In this manner, related art means for driving a CCD discharges at high speed the electric charges in the vertical shift registers 2 in the period V3 when the record trigger is outputted. Then, the means stores the electric charges in the photodiodes 1 in the time interval tb4 included in the next period V4, and further, closes the light shading element 12 for transferring the electric charges in the next period V5. Furthermore, the means opens the light shading element 12 again in the subsequent period V6.
However, response time is necessary to close the light shading element 12 in the above-mentioned related art means for driving a CCD. Therefore, even if a close command is issued at the starting end of the period V5, the time interval tm is necessary to completely achieve the closing operation. Owing to this, the CCD 13 is exposed even in the period V5 when the electric charges are transferred.
Therefore, since the electric charges are generated in the photodiodes 1, and particularly, in case a subject has high intensity, a part of the generated electric charges may be mixed in transfer parts of the vertical shift registers 2 even if the electric charges are vertically discharged with using the sub-pulses SUB. Hence, some smears may be superposed on an original CCD signal.
This is not limited to the time interval tm when the electric charges are generated, but, even after the light shading element 12 is completely closed, the electric charges generated in the time interval tm may remain behind in the substrate of the photodiodes 1. Therefore, the residual electric charges may be transferred by the vertical shift registers 2. In addition, although the smears in the transfer parts can be removed for a while, the smears mixed in the transfer operation in the period V4 can not be removed.